The present invention relates generally to tilt mechanisms for chairs, and in particular, to a tilt assembly, and method therefore, which provides a variable biasing force for a user as the user reclines in the chair.
Chairs of the type typically used in offices and the like are usually configured to allow tilting of a body support member, such as a seat or backrest. Often, such chairs are configured as synchrotilt chairs, wherein the backrest and seat tilt simultaneously, but at different rates. Generally, it is desirable to have the backrest tilt at a slightly greater rate so as to allow the body cavity of the chair to open.
Typically, the tilting of such office chairs is controlled by one or more springs, which act against the weight of the user and bias the seat and/or backrest in an upward direction. In general, the resistive force or moment required to support the user may increase as the user reclines in the chair, since the center of gravity of the user tends to move rearwardly as they recline.
While the resistive force of the spring can be adjusted by preloading the spring, each spring is characterized by a spring rate, or spring constant. Accordingly, preloading does not necessarily alter the characteristics of the spring in the range of tilting. Typically, the user adjusts the pre-load of the spring with an actuator, such as a knob, when the backrest and seat are in an upright position. Due to the nature of the spring, it may take a large number of turns of the actuator, e.g., in the neighborhood of 15-20 revolutions, to adjust the spring when switching from a light user to a heavier user. Moreover, as the user tilts rearwardly in the chair, the spring rate may not correspond to the force needed to counter the shift in the center of gravity of the user. Therefore, chairs often will be equipped with spring systems that are not suited for or capable of responding to the need for an increased biasing force or moment when the chair is in a more reclined position. Moreover, because of the requisite size of the springs, the mechanisms used to adjust the amount of initial resistive force or torque, i.e., preload, can be difficult to actuate, and can be progressively more difficult to adjust as higher settings are reached.
Briefly stated, the invention is directed to an improved tilt assembly for a chair. In one aspect of the invention, the tilt assembly comprises a tilt housing, a body support member pivotally attached to the tilt housing about a first horizontal axis and a spring moveably mounted to said tilt housing at a second horizontal axis. The body support member, which is preferably a back support member, is moveable between at least a first and second position, with the second position being at a greater rearward tilt angle than the first position. Preferably, the second horizontal axis is adjustably moveably relative to said first horizontal axis. The spring biases the body support member at a first location when the body support member is in the first position and at a second location when the body support member is in the second position. Preferably, the second location is a greater distance from the first horizontal axis than is the first location.
In a preferred embodiment, the spring is mounted to a spring housing, which is moveably mounted to the tilt housing. In a more preferred embodiment, an adjustment mechanism is disposed in the tilt housing and engages the spring housing. The adjustment mechanism engages the spring housing and is operable to move the spring housing relative to the tilt housing. Preferably, the adjustment mechanism comprises an engagement member engaged with the spring housing and an actuation shaft threadably engaged with the engagement member.
In another aspect, a method for providing a variable biasing force for a body support member is provided. The method comprises providing a tilt housing, a body support member pivotally attached to the body support member about a first horizontal axis and a spring moveably mounted to the tilt housing at second horizontal axis spaced from said first horizontal axis. The method further preferably comprises moving the second horizontal axis to a desired position relative to the first horizontal axis, tilting the body support member from a first position to a second position, biasing the body support member with the spring at a first location when the body support member is in the first position, and biasing the body support member at a second location when the body support member is in the second position, wherein the second location is a greater distance from the first horizontal axis than is the first location.
In a preferred embodiment, the method further comprises moving the second horizontal axis from a first desired position to a second desired position relative to said first horizontal axis, wherein the first location of biasing said body support member with said spring is moved a corresponding amount.
The present invention provides significant advantages over other tilt assemblies. For example, the invention provides for a variable effective spring rate, which results in an increasing biasing moment as the user reclines in the chair. As a result, the tilt assembly is able to provide an ever increasing biasing moment to balance and offset the increasing loads applied by the user as they tilt rearwardly in the chair. In particular, the location where the spring biases the body support member moves away from the first horizontal axis about which the body support member pivots so as to thereby increase the moment arm. As a result, the moment applied by the spring about the first horizontal axis increases with the increase of the moment arm. Accordingly, the biasing moment applied to support the user is thereby increased to counter the increased moment applied by the user about the first horizontal axis as the user tilts rearwardly in the chair.
Another significant advantage of this invention is that the initial resistive force of the spring can be easily adjusted simply by moving the second horizontal axis at which the spring is mounted, relative to the first horizontal axis. This adjustment does not require prestressing the spring at differing levels. Accordingly, the adjustment mechanism, which is operable to move the second horizontal axis relative to the first horizontal axis, can be easily manipulated without progressive difficulty.
The present invention, together with further objects and advantages, will be best understood by reference to the following detailed description taken in conjunction with the accompanying drawings.